Table of contents

New measurements of the transition energies of the principal series of helium are reported. They were obtained in a high resolution photoabsorption experiment with synchrotron radiation. It is shown that the wavelength measurements above n=10 are consistent with the more accurate quantum defect extrapolation based on the first nine members to an experimental precision of +or-0.00042 AA.

The authors have shown that cross correlation between the fluctuating phases of two lasers (as introduced by Wong and Eberly (1977) and Dalton and Knight (1982-3)) produces significant effects on the shape and asymmetry of the optical double-resonance spectra. If suitable sources can be found, these features should be readily observable.

Simultaneous two-electron (K+L) one-photon excitations in cobalt were studied using X-ray absorption spectroscopy. The cross sections measured, 0.29% of that of the one (K) electron one-photon excitation, are in very good agreement with the authors' previous measurements on Ni. The two-electron excitation energies are about 30 eV higher than the sum of those required for exciting the electrons involved independently. The theoretical energy levels obtained by Delta SCF calculations are found to be higher than the measured ones by as much as 55 eV, and density functional theories of core-hole excitations.

Selective production and storage at thermal energies of highly charged ions (e.g. hydrogen-like argon, Ar¹⁷⁺) are imminently feasible. Multi-step photoionisation by wiggler-enhanced synchrotron radiation of thermal ions stored in a Penning trap is discussed. Potential applications of these ions to electron transfer and photoionisation measurements are pointed out.

Rotational constants are reported for the nu =1 and 2 levels of the ² Pi _3/2 substrate of ²⁰²HgD. These values should prove useful in understanding previously noted anomalies in the NgH/HgD spectra.

It is shown how to obtain the zero-field cross section from the existing treatments of potential scattering in strong, quantising magnetic fields. Weak-field corrections to the zero-field transmission matrix elements are derived as well.

Relative cross sections for single electron capture into selected n=3, l, m_l substates are determined by VUV spectroscopy for C⁴⁺-H₂ collisions in the velocity range 0.2 to 0.4 au. For capture to the 3p state, the m_l=0 substrate is predominantly populated at low velocity, but with increasing velocity, the population of the m₁=+or-1 substrates tends to dominate.

Calculations have been made of the p-wave contributions to the elastic scattering and positronium formation cross sections for positron-hydrogen scattering in the ore gap, 6.8-10.2 eV. The Kohn and inverse Kohn variational methods were used with trial functions containing as many as 84 short-range correlation terms of each symmetry, and there is considerable evidence that the most accurate of the results obtained are close to being fully converged.

The l^N configuration is described as a collection of ¹/₂N(N-1) electron pairs, with the angular momenta of the electrons of each pair being coupled to the (2l+1) SL terms of l². In this way, a set of parameters is defined which can be visualised as the energies of these particular SL terms. The parameters represent the effects of two-particle excitations on the l^N configuration to second order. Moreover, they turn out, after a minor modification, to form an orthogonal set. The number of pairs multiplied by the average pair energy proves to be equal to the average energy of the full l^N configuration. Inner products of general two-particle operators are calculated and conditions for their orthogonality are derived. For the d^N configurations, two orthogonal three-body operators are added in order to obtain a complete set of electrostatic parameters. A simple general procedure is sketched that allows for the determination of the contributions of physical excitations to the various orthogonal parameters. This is illustrated by the s to d case. Values of all the matrix elements used are given.

A relatively simple approximate ab initio method for the evaluation of correlation effects on atomic energy differences is developed. In this method, which is a modification of the semiempirical version of Sinanoglu's non-closed-shell many-electron theory (NCMET), the all-external contributions to the correlation energies are evaluated by a scheme based on the knowledge of second-order pair energies calculated for approximate closed-shell atomic systems. The present method can be applied to a much larger set of atomic systems than the semiempirical NCMET. To test the reliability of the present approach extensive calculations are performed for energy differences involving states of the 1s²2s^m2pⁿ configurations for various atomic numbers. These calculations could be to a large extent based on previously published values of both the 'charge' contributions to the energies and the relevant pair energies. The results for (i) term splittings within a single configuration for neutral atoms as well as positive and negative ions, (ii) excitation energies, (iii) ionisation potentials and (iv) electron affinities prove that the present approach can be used for reliable predictions of atomic energy differences. Since the method is non-relativistic, some attempts have been made to account for the relativistic corrections. In all cases considered the inclusion of these corrections has further improved the agreement with the experimental results.

The result of an experimental evaluation of the K-shell fluorescence yield, omega _K, of monocrystalline Ge is reported. The procedure makes use of the information contained in the escape peaks which are present in the characteristic X-ray spectra given by an HP Ge detector. Details of the experimental methodology, peak area evaluations and uncertainty treatments are described. A value omega _K=0.549+or-0.011 is obtained and compared with recent values obtained from experiment, theory, interpolation or normalisation. The authors consider the effects of the outer-shell configuration and the use of theoretical photoelectric cross section values.

The absorption of two identical photon by a charged particle in a uniform magnetic field is studied within the framework of non-relativistic quantum electrodynamics. The matrix elements for absorption are expanded into multipole moments, and the two-dimensional dipole approximation is then made. The authors show that non-relativistic theory can only be used to describe absorption into the first Landau level above the initial state. It is also shown that making the 2D approximation at the start of the calculation leads to a different result, since this approach includes some contributions to the matrix element that should be included. The angular dependence of the transition rate is given for various polarisations of the incident radiation.

The Wannier threshold energy dependence of cross sections and angular distributions are derived for the Coulomb break-up of three-particle systems with arbitrary angular momentum. Within the introduction of relative (Jacobi) coordinates, the coupling of internal degrees of freedom within a Wannier mode is considered in detail.

Doubly differential cross sections for delta -electron ejection in proton-rare-gas collisions have been studied at ejection angles of 50, 90 and 120 degrees , electron energies of about 10 to 550 eV, and incident energies around 300 keV. Performing a charge-state analysis of the recoil ion in coincidence with the delta electrons we were able to distinguish between single and multiply ionising collisions. It was observed that the relative importance of multiple ionisation events is smaller at forward compared with backward ejection angles and increases with increasing energy of the delta electron. An analysis in terms of an ionisation model of independent electrons gives fair agreement with the experimental data.

Using Born's approximation the authors have analysed the structure of the doubly differential cross section for electron loss from fast H^- near the forward direction. They show that electrons ejected with velocities slightly smaller than the projectile, originate in collisions in which the 'diffuse' electron of H^- scatters as a nearly-free electron and excites (ionises) the target. Electrons ejected with velocities close to that of the projectile come from three sources: (i) double-electron loss in which the final state is strongly influenced by the Coulomb interaction with the resulting fast proton; (ii) single electron loss in which the target is excited by the H core, while the ejected 'diffuse' electron acts as a separator; and (iii) single detachment with no excitation of the target which comprises a small fraction of the events for He targets, but becomes important for heavier targets such as Ar.

A model is developed where the implicit continuum intermediate states are approached by Coulombic phases (such that the correct boundary conditions are satisfied) in electron capture ion-atom collisions. This model can be seen as a symmetrisation of the eikonal and Glauber ones. Close agreement is found with continuum distorted-wave and second born approximations when differential cross sections are calculated at small angles and high energies. Experimental total cross sections are well reproduced for a wide energetic range for H⁺+H and H⁺+He collisions.

Conservation of the electronic configuration of ground-state and metastable ions in the capture of an electron from Li(2s) into excited states has been utilised to measure metastable ion beam fractions for multiply charged ions. Results are presented for beams of C²⁺, N²⁺, N³⁺ and O⁴⁺ respectively, which have been produced by means of an ECR ion source.

Results are given for the low-energy elastic scattering of electrons from krypton and xenon. The calculations were carried out by treating exchange exactly and including only the dipole part of the polarisation potential. Values are given for the phaseshifts, differential, total elastic and momentum transfer cross sections. Comparison are given with recent experimental and theoretical work for these processes.

Differential cross sections were measured for the scattering of 10, 20 and 200 eV electrons by rubidium atoms. Measurements were carried out at angles ranging from 5 to 120 degrees to obtain elastic cross sections as well as inelastic cross sections for 5²P and (4²D+6²S) excitations. The measured relative cross sections were brought onto an absolute scale through a normalisation procedure utilising the optical oscillator strength for the resonance transition of rubidium. Integral and momentum cross sections for the 5²P excitation were obtained by extrapolating the differential cross sections to 0 and to 180 degrees . (Present results are compared with integral cross sections resulting from previous measurements of optical excitation functions).

Electron impact double ionisation cross sections for Ca and Sr have been calculated in the modified binary encounter model. Contributions of inner-shell ionisation and Auger emission have been included in the calculations. The calculated cross sections are in good agreement with the experimental observations of McFarland (1968) and Okudaira (1970). The probable reasons behind the discrepancies observed between the experiments and the calculated cross sections have been discussed.

The electron-polarised-photon coincidence technique has been used to study the X¹ Sigma _g⁺ to C¹ Pi _U to X¹ Sigma _g⁺ excitation/de-excitation process in H₂ and D₂. Polarisation correlations for integrated vibrational bands have been measured for electron scattering angles up to 25 degrees and incident electron energies up to 80 eV. It was found that the P(0) parameter displays a pronounced dip to zero at a scattering angle of theta _c-12.5 degrees in H₂, independent of the actual impact energy, and at theta _c=10 degrees in D₂. Characteristic features in the P(¹/₄ pi ) angular dependence are a negative value at a small scattering angle, P(¹/₄ pi )=-0.36 at theta _c=10 degrees in hydrogen and P(¹/₄ pi )=-0.23 at theta _e=7.5 degrees in deuterium, and a rapid variation of P(¹/₄ pi ) back to zero. The region of the steepest gradient in the P(¹/₄ pi ) curve was found to coincide with the dip in P(0). The decline in the level of coherence when going from H₂ to D₂ can be attributed to the more developed rotational envelope in deuterium. An attempt is made to relate the observed systematic trends in the data to the properties of the respective target molecule.

Ab initio results for e-LiH and e-CO Scattering are reported using a new numerical continuum basis in the R-matrix method. In the case of e-LiH scattering, converged ² Sigma eigenphase sums are obtained in the static exchange approximation. In the case of e-CO scattering, converged eigenphase sums and momentum transfer cross sections are obtained at a low and intermediate energies including electron exchange and nonadiabatic polarisation effects. The low-energy e-CO ² pi shape-resonance parameters are calculated as a function of internuclear separation for the first time, and are found to be in excellent agreement with experiment. Evidence is also presented for the e-CO ² Sigma shape resonance at about 20 eV. The momentum transfer cross section is in good agreement with experiment.

An electron energy-loss spectrometer incorporating a position-sensitive multidetector and differential pumping has been used to study the spectrum of hydrogen chloride. A combination of low incident electron energies and large scattering angles has been used for the first time to study transitions in this molecule. Energy-loss spectra obtained under a variety of experimental conditions are presented in the range 9-16.5 eV. Many new transitions are observed and from a consideration of the dependence of their intensity on incident energy and scattering angle some are characterised as being electric-dipole forbidden. A number of new allowed transitions are observed and the energy locations of many previously reported transitions are also verified. In order to assist analysis of structure observed in the region 12.7-16.5 eV, spectra of deuterium chloride have also been obtained. Three Rydberg progressions are observed in HCl in the region 9.3-12.8 eV and the excitation energies of these transitions are used to determine values for the two components of the first ionisation potential.

For pt.I see ibid., vol.17, p.1675 (1984). A theory of stepwise excitation of atoms is presented in which electron excitation is followed by strong single-mode laser excitation. A model of the strong laser excitation processes is considered and theoretical expressions are obtained for line polarizations and electron-photon coincidence signals applicable to both axial and planar symmetry stepwise excitation experiments.